Processing characteristics are critical in assessing the commercial viability of polyurethane elastomers. Examples of these processing characteristics are the pot life, gel time, demold time, and green strength, among others. A commercially useful pot life is necessary to enable sufficient working time to mix and degas where necessary, the reactive polyurethane forming components. Gel time is critical in enabling complete filling of molds before gelation occurs, particularly when large, complex molds are utilized, while demold time is important in maximizing part production. Too long a demold time necessitates larger numbers of relatively expensive molds for a given part output. Demold time is especially critical for glycol extended elastomers which tend to be slow curing. These requirements are often competing. For example, a decrease in catalyst level will generally result in longer pot life and increased gel time, but will often render demold time unsatisfactory, and vice versa.
Green strength is also important. Green strength is a partially subjective measure of the durability of a polyurethane part immediately following demold. The characteristics of the polyurethane forming reaction is such that full strength of the polyurethane part does not develop for a considerable time after casting. The partially cured, or "green" part must nevertheless be demolded within a reasonable time. Polyurethane parts typically display two types of "poor" green strength. One type is such that the part is gelled and rigid, but is brittle and easily torn. Those normally skilled in the art of polyurethane elastomers refer to this type of poor green strength as "cheesy" in reference to its "cheese-like" consistency. The other type of "poor" green strength is when the part is soft and pliable, and permanently distorts during the demolding process. By contrast, parts which upon demold display durability and which can be twisted or bent without permanent damage are said to possess "excellent" green strength. While demold time limits production, poor green strength increases scrap rate.
Various methods of increasing green strength and decreasing demold time have been examined. Increasing catalyst level, for example, may often desirably influence these properties. However, as previously stated, increased catalyst levels also decrease both pot life and gel time. Moreover, when microcellular elastomers are to be produced, some catalysts increase the isocyanate/water reaction to a greater degree than the isocyanate/polyol reaction, and thus can affect processability.
It is well known in the art that polyurea and polyurethane/urea elastomers are much easier to process than all urethane elastomers. Polyurea and polyurethane/urea elastomers are prepared using amine-terminated polyols and/or diamine chain extenders. The most common urethane/urea elastomer system uses a toluene diisocyanate prepolymer reacted with the diamine extenders, methylene-bis-(2-chloroaniline), better known as MOCA or MBOCA. This system is known to give a long pot life (10 to 20 minutes) with commercially acceptable demold times of less than 60 minutes with excellent green strength. In addition to this, there is minimal sensitivity to changes in processing conditions with this system. However, some of the physical properties of the elastomers containing urea linkages are inferior compared to all urethane elastomers (i.e. softness, tear strength, resilience and hydrolysis resistance). Other common diamine chain extenders may unduly affect pot life and gel time, however.
Primary hydroxyl-containing polyols have also been used to decrease demold time and improve green strength with some success, particularly in RIM applications. However, in general, reliance on high primary hydroxyl polyols causes a decrease in pot life and gel time and moreover, may make the elastomers more subject to adsorption of water due to the more hydrophilic nature of the polyoxyethylene cap which provides the primary hydroxyl content. Elastomers based on primary hydroxyl polyols are also generally harder than those prepared from polyoxypropylene homopolymer polyols.
In U.S. Pat. No. 5,106,874 is disclosed the use of polyoxyethylene capped polyoxypropylene polyether polyols in diamine extended polyurethane/urea elastomers where the polyol is prepared using alkali metal catalysts and low temperatures to minimize polyol unsaturation. The '874 patentees indicate that when ethylene oxide capped polyols having an unsaturation of 0.04 meq/g polyol are used in the preparation of diamine extended elastomers, demold time and green strength improve. However, the systems disclosed are rigid polyurethane/urea elastomers with a high proportion of urea linkages only suitable for RIM, as demold times are on the order of 30-40 seconds.
Polyol unsaturation and its effect on polyurethane properties has been commented upon at great length, although the effects are unpredictable and difficult to quantify. The relationship of unsaturation to processing has not been studied to any great degree. During synthesis of polyoxypropylene polyols by base catalyzed oxypropylation of a suitable polyhydric initiator, a competing rearrangement produces monohydric allyloxy species which are in turn oxypropylated. The mechanism of unsaturat on formation has been discussed, for example, in Block and Graft Polymerization, Vol. 2, Ceresa, Ed., John Wiley & Sons, pp. 17-21. Whatever the source of terminal unsaturation, it is well known that the mol percent of terminally unsaturated monol increaser rapidly with increasing molecular weight of the polyhydric species. Thus, while very low molecular weight, conventionally catalyzed, polyoxypropylene glycols of 200 Da to 500 Da equivalent weight may have low monol content, for example less than about 1 mol percent, a similarly prepared diol of 2000 equivalent weight may contain 45 mol percent to 50 mol percent of monol. This large increase in monol content lowers the nominal functionality of two to an average functionality of c.a. 1.6 or less.
Polyol unsaturation is generally measured by titration in accordance with ASTM test method D-2849-69 or its equivalent, and is expressed in milliequivalents of unsaturation per gram of polyol, hereinafter, "meq/g". Traditional, base-catalyzed polyols in the moderate to higher equivalent weight range, for example from 1000 Da to 2000 Da equivalent weight, generally have unsaturations in the range of 0.03 to about 0.095 meq/g.
To lower the unsaturation, and thus the monol content, various process parameters have been adjusted, such as the catalyst level and oxyalkylation temperature. However, improvement in the level of unsaturation in such cases comes at the expense of process time. Moreover, the improvement is at best marginal. Use of alternative catalyst systems, such as barium hydroxide, transparent iron oxides, diethyl zinc, metal phthalocyanines, and combinations of metal naphthenates and tertiary amines have also been proposed, the latter method being able to reduce unsaturation to the range of 0.03 to 0.04 meq/g in c.a. 4000 Da polyoxypropylene triols. However, even at this lower level, as compared to the 0.07 to 1.0 meq/g representative of conventionally catalyzed but otherwise similar polyols, the mol percent monol is still high, for example 25 mol percent or thereabouts.
Significant improvement in monol content of polyoxypropylene polyols has been achieved using double metal cyanide catalysts, for example the non-stoichio-metric zinc hexacyanocobaltate.glyme catalysts disclosed in U.S. Pat. No. 5,158,922. Through use of such catalysts, polyoxypropylene polyols of much higher molecular weight than previously thought possible have been prepared, for example 10,000 Da polyoxypropylene triols with unsaturations of 0.017 meq/g. J. W. Reish et al., "Polyurethane Sealants and Cast Elastomers With Superior Physical Properties", 33RD ANNUAL POLYURETHANE MARKETING CONFERENCE, Sep. 30-Oct. 3, 1990, pp. 368-374.
Numerous patents have addressed the use of higher molecular weight polyols to prepare polyurethanes. In such cases, the improvements are said to result either solely from the ability to provide higher molecular weight polyols of useful functionality, or additionally, the low monol content, the monol thought to react as "chain-stoppers" during polyurethane addition polymerization. Illustrative examples of such patents are U.S. Pat. No. 5,124,425 (room temperature cure sealants from high molecular weight polyols having less than 0.07 meq/g unsaturation); U.S. Pat. No. 5,100,997 (diamine extended polyurethane/urea elastomers from high molecular weight polyols having less than 0.06 meq/g unsaturation); U.S. Pat. No. 5,116,931 (thermoset elastomers from double metal cyanide catalyzed polyols having less than 0.04 meq/g unsaturation); U.S. Pat. No. 5,250,582 (high molecular weight DMC.glyme catalyzed polyols grafted with unsaturated polycarboxylic acids to provide in situ blowing agent); U.S. Pat. No. 5,100,922 (high molecular weight polyols, preferably DMC.glyme catalyzed, together with aromatic crosslinking agent useful in preparing integral skin foams); U.S. Pat. No. 5,300,535 (high molecular weight polyols with unsaturation less than 0.07 meq/g useful in preparing foams with low resonant frequencies for seating applications); and U.S. Pat. No. 4,239,879 (elastomers based on high equivalent weight polyols) However, none of these patents address processing characteristics, which are of paramount importance in the cast elastomer industry.
C. P. Smith et al., in "Thermoplastic Polyurethane Elastomers Made From High Molecular Weight Poly-L.TM. Polyols", POLYURETHANES WORLD CONGRESS 1991, Sep. 24-26, 1991, pp. 313-318, discloses thermoplastic elastomers (TPU) prepared from polyoxyethylene capped polyoxypropylene diols with unsaturation in the range of 0.014-0.018 meq/g. The polyols used were prepared using double metal cyanide.glyme catalysts, and the elastomers showed increased physical properties as compared to elastomers prepared from a conventionally catalyzed diol of 0.08 meq/g unsaturation. Additional examples of low unsaturation polyols in polyurethane elastomers is given in "Comparison of the Dynamic Properties of Polyurethane Elastomers Based on Low Unsaturation Polyoxypropylene Glycols and Poly(tetramethylene oxide) Glycols", A. T. Chen et al., POLYURETHANES WORLD CONGRESS 1993, Oct. 10-13, 1993, pp. 388-399. Cited as positively influencing elastomer physical properties were the low monol content and low polydispersity of the c.a. 0.015 meq/g, DMC.glyme catalyzed polyols used. Neither publication addresses processing characteristics, or the surprising effect that ultra-low unsaturation and broad polydispersity have on these characteristics.
It has been reported that low unsaturation polyols sometimes produce polyurethanes with anomalous properties. For example, the substitution of a low unsaturation 10,000 Da molecular weight triol for a 6000 Da molecular weight conventionally catalyzed triol produced an elastomer of higher Shore A hardness where one would expect a softer elastomer, whereas substitution of a similarly DMC.glyme catalyzed 6000 Da molecular weight triol for a conventional 6000 Da molecular weight triol showed no increase in hardness. R. L. Mascioli, "Urethane Applications for Novel High Molecular Weight Polyols", 32ND ANNUAL POLYURETHANE TECHNICAL/MARKETING CONFERENCE, Oct. 1-4, 1989. Moreover, and as more fully set forth below, butanediol extended elastomers prepared from DMC.glyme catalyzed polyols exhibited demold times of 150 minutes or more, which is commercially unacceptable in cast elastomer applications.
In copending U.S. application Ser. No. 08/156,534, U.S. Pat. No. 5,470,813, herein incorporated by reference, are disclosed novel double metal cyanide.t-butanol (DMC.TBA) catalysts prepared by intimate mixing of catalyst reactants. These catalysts lack the crystallinity characteristic of DMC.glyme catalysts as shown by X-ray diffraction studies, and moreover exhibit threefold to tenfold higher activity in propylene oxide polymerization. It is especially surprising that the unsaturation is lowered to unprecedented ultra-low values through use of these catalysts, with measured unsaturations of from 0.003 meq/g to 0.007 meq/g routinely achieved.
While the measurable unsaturation implies an exceptionally low but finite monol content, it is especially surprising that analysis of the product polyols by gel permeation chromatography showed no detectable low molecular weight fraction. The polyols are essentially monodisperse. The virtually complete absence of any low molecular weight species renders such polyols different in kind from even those prepared from DMC.glyme catalysts.